fix(speak): explicit imports for voice-quota helpers

Nitro auto-import doesn't reliably pick up named exports from
db/voiceQuota.ts at runtime — speak endpoint threw 500 with
"ReferenceError: getRemainingVoiceQuota is not defined".

Explicit imports for getRemainingVoiceQuota, consumeVoiceQuota,
estimateAudioSeconds + getPlanLimits.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/server/api/coach/speak.post.ts

@@ -1,5 +1,11 @@
 import type { H3Event } from "h3";
 import type { VoiceConfig } from "../../utils/plan-features";
+import { getPlanLimits } from "../../utils/plan-features";
+import {
+  getRemainingVoiceQuota,
+  consumeVoiceQuota,
+  estimateAudioSeconds,
+} from "../../db/voiceQuota";
 
 /**
  * POST /api/coach/speak